Field of the Invention
The present invention relates to a hydraulic drive system.
Background Information
Machinery, such as hydraulic shovels and wheel loaders, is provided with a work implement that is driven by a hydraulic cylinder. The hydraulic cylinder is supplied with the hydraulic fluid discharged from a hydraulic pump. The hydraulic fluid is supplied to the hydraulic cylinder by way of a hydraulic circuit. For instance, Japanese Laid-Open Patent Application Publication No. 2003-21104 proposes a work implement equipped with a closed hydraulic circuit for supplying hydraulic fluid to the hydraulic cylinder. The closed hydraulic circuit regenerates the positional energy of the work implement. Thus, it is possible to reduce the fuel consumption of a prime mover driving the hydraulic pump.
The above-mentioned closed hydraulic circuit is also provided with a relief valve. The relief valve opens when hydraulic pressure in the closed hydraulic circuit is at or above a predetermined relief pressure. Thereby, the relief valve regulates the increase in hydraulic pressure in the closed hydraulic circuit.